- Cyclical pressurization of cultured chondrocytes results in increases in cyclic AMP and in the rate of proteoglycan synthesis. Intermittent increases in hydrostatic pressure are also associated with hyperpolarization of chondrocyte cell membranes and activation of Ca²⁺-dependent K+-ion channels but the physiological basis for this response to mechanical stimulation is unclear.
- Experiments have been undertaken to better define the types of ion channels involved and to explore the possibility that the hyperpolarization response associated with cyclical pressurization of chondrocytes follows activation of stretch-activated ion channels.
- The mean membrane potential of chondrocytes in non-confluent monolayer cell culture rose from −15.3 ± 0.24 mV to −21.1 ± 0.28 mV (n = 60, P < 0.0001) after intermittent pressurization (0.33 Hz, 16 kPa, 20 min).
- Strain gauge measurements showed that cyclical pressurization was associated with strain on the base of the culture plate. The amplitude of the hyperpolarization response was proportional to the microstrain to which cells were subjected.
- Membrane hyperpolarization did not occur when chondrocytes were subjected to cyclical pressurization in rigid glass culture dishes or plastic dishes positioned in the pressurization chamber so as to avoid bending of the base of the culture dish.
- Indirect evidence that the hyperpolarization response after intermittent pressure-induced strain was associated with stimulation of stretch-activated ion channels was obtained from experiments with gadolinium, amiloride and hexamethylene amiloride, each of which abolished hyperpolarization.
- Experiments with apamin, charybdotoxin and iberiotoxin showed that the Ca²⁺-activated K+ channels involved in the hyperpolarization response are apamin-sensitive, charybdotoxin- and iberiotoxin-resistant, low-conductance channels.
- Somatostatin and cadmium chloride, which block L-type calcium channels, abolished strain-induced chondrocyte hyperpolarization. EGTA, which chelates extracellular Ca²⁺, reduced the response to 48% of control values, and thapsigargin, which raises intracellular Ca²⁺ by inhibition of Ca²⁺—ATPase in endoplasmic reticulum, caused hyperpolarization independently with further hyperpolarization after pressure-induced strain. These data indicate that chondrocyte hyperpolarization was dependent on intracellular Ca²⁺ concentrations.
- Further work is required to determine whether stretch-activated ion channels shown to be associated with chondrocyte hyperpolarization after cyclical pressure-induced strain are also involved in the signal transduction process that leads to increases in proteoglycan synthesis.
Keywords:
articular; cartilage; cells; ion channels; mechanoreceptors; membrane potential